The present invention relates to optimizing a charged particle beam device, such as a focused ion beam (FIB) device, and in particular to optimizing the chromatic aberration in a FIB using an energy filter.
Liquid metal ion source (LMIS) based focused ion beam (FIB) systems are an important, sometimes indispensable tool, in many branches of science and industry, especially in the semiconductor industry. The applications of current FIB systems include device modification, failure analysis, probe point creation, photomask repair, maskless lithography, transmission electron microscope (TEM) sample preparation, scanning ion microscopy and secondary ion mass spectroscopy.
Current applications of FIB systems require a high resolution, i.e., small beam diameter, and high current density. As is well known, the central part of a FIB system is the column, which consists of a LMIS and charged particle optical components which perform extracting, transporting, deflecting and focusing of the ion beam. FIB systems are well known, see for example, U.S. Pat. No. 5,825,035, which is incorporated herein by reference.
The resolution of a conventional column for most useful beam current ranges is limited in the form of chromatic aberration due to the large energy spread of the LMIS, which is in a range of 5 eV-10 eV. Because the energy spread is intrinsic in LMISs, there is no effective way to reduce the energy spread further by tuning the emission of the LMIS. In general, the characteristics of the column dominate the overall FIB system performance. Thus, the intrinsic energy spread of the LMIS significantly affects the performance of a conventional FIB system.
In accordance with the present invention, a charged particle beam is passed through an energy filter to alleviate the chromatic aberration. A charged particle source, such as a LMIS is used to produce charged particles. An energy filter downstream of the charged particle source reduces the energy spread of the charged particle beam by passing only the charged particles that are within a desired energy range. By reducing the energy spread of the charged particle beam, the chromatic aberration of the system is reduced, thereby increasing the current density of the beam.
The energy filter may be, e.g., a Wien filter that is optimized for energy filtering purposes. For example, the energy filter may use a quadrupole structure between two magnetic pole pieces thereby producing a combined quadrupole electric field and dipole electric field within a magnetic field. The quadrupole structure may be four hyperbolic surfaces or, e.g., four cylindrical surfaces with axes that run parallel to the direction of travel of the charged particle beam. With the proper application of voltages on the quadrupole structure, a combined quadrupole electric field and dipole electric field may be generated and which will act as the desired energy filter.